The unidirectional influx of methionine (Jmc) into the brush border epithelium of chicken jejunum has been studied in tissues mounted in a chamber so that only the mucosa was exposed to tracer. Jejuna leached of Na ion transport methionine from a medium devoid of Na ion with reduced apparent affinity (Kt) and maximal flux (Jmax mc). Addition of Na ion to the medium for one min restored Kt but only partially restored Jmax mc. Even after a 30 min exposure to Na ion, no further change in Jmax mc occurred. Theophylline was found to mantain Jmax mc in the absence of Na ion. Essentially complete restoration of Kt and Jmax mc could be attained when theophylline-treated tissue was exposed to Na ion for 30 min. At low substrate levels (micron Molar), essentially complete inhibition of Jmc was brought about by leucine and alanine; however, glycine and proline maximally inhibited 1/2 - 2/3 of the flux. Alpha aminoisobutyric acid inhibited no more than 1/4 of Jmc irrespective ofthe inhibitor concentation tested. At 5 mM substrate, only leucine and alanine of the substances tested inhibited influx. All inhibitions were of the classical competitive type. These findings suggest that multiple systems function in Jmc that are shared or unshared with the various inhibitory amino acids. If this is the case, then such processes must have similar kinetic constants and Na ion sensitivies, because analysis of influx by the Lineweaver-Burk method showed a single function both in the presence and absence of Na ion when the flux was measured over a wide concentration range (10 micron M - 20 mM). A positive correlation was found in the ability of a series of neutral amino acids to inhibit Jmc in the presence of Na ion, in its absence and in theophylline-treated tissues. Similar studies are being conducted with the substrate glycine.